1. Field of the Invention
This invention relates to an electro-optic device, a drive substrate for an electro-optic device and a method of manufacturing these devices, and in particular, relates to a structure and method suitable for an LCD display device comprising a dual gate thin film insulating gate field effect transistor (hereafter referred to as dual gate MOSTFT) using a monocrystalline silicon layer grown heteroepitaxially on an insulating substrate as the active region, and a passive region.
2. Description of Related Art
Examples of active matrix LCD displays are a device comprising a display unit using amorphous silicon as TFT and external drive circuit IC, a one piece construction comprising a display unit and drive circuit using polycrystalline silicon obtained by solid phase growth as a TFT (Japanese Patent Application Laid-Open No. Hei 6-242433), and a one piece construction of a display unit and drive circuit using polycrystalline silicon subjected to excimer laser annealing as a TFT (Japanese Patent Application Laid-Open No. Hei 7-131030).
However, although productivity of conventional amorphous silicon TFT is good, their electron mobility is as low as about 0.5-1.0 cm2/vxc2x7sec, so a p channel MOSTFT (hereafter referred to as PMOSTFT) could not be manufactured. Therefore, as a peripheral drive circuit unit using a pMOSTFT cannot be formed on the same glass substrate as a display unit, the driver IC had to be external and was installed by a TAB method or the like, so it was difficult to reduce costs. For this reason, there was a limit to the degree of high precision that could be achieved. Further, as the electron mobility is only about 0.5-1.0 cm2/vxc2x7sec, sufficient ON current cannot be obtained, and if the transistor is used for a display unit, the transistor size necessarily becomes large which is a disadvantage for obtaining a high pixel aperture.
As the electron mobility of a related art polycrystalline TFT is 70-100 cm2/vxc2x7sec and can also support high precision, LCD (liquid crystal display devices) using polycrystalline silicon TFT formed in one piece with a drive circuit have recently been attracting attention.
However, in the case of a large LCD of 15 inches or more, as the electron mobility of polycrystalline silicon is 70-100 cm2/vxc2x7sec, driving performance is insufficient so an external drive circuit IC was still needed.
In the case of a TFT using polycrystalline silicon formed as a film by solid phase growth, annealing for more than 10 hours at 600xc2x0 C. or higher, and the formation of a gate SiO2 by high temperature oxidation at about 1000xc2x0 C., are necessary, so a semiconductor manufacturing device has to be used. Consequently, a wafer size of 8-12 inches diameter is the limit, highly heat resistant, costly quartz glass has to be used, and it is difficult to reduce costs. Therefore, usage in this case is limited to EVF or data/AV projectors.
Further, in related art polycrystalline silicon TFT obtained by annealing with an excimer laser, there are many problems such as stability of the excimer laser output, productivity, increase of device cost due to larger size, and decrease of yield/quality.
These problems are more severe in the case of large glass substrates of a 1 m square, and it is even more difficult to improve performance/quality and reduce costs.
It is therefore an object of this invention to enable manufacture of an active matrix substrate, in particular with regard to a peripheral drive circuit unit, comprising a built-in high-performance driver by uniformly forming a film of a monocrystalline silicon layer having a high electron/hole mobility at relatively low temperature and an electro-optic device such as a thin film semiconductor for a display using this substrate, to enable manufacture of a one-piece construction comprising a display unit comprising an channel MOSTFT (hereafter referred to as nMOSTFT), pMOSTFT or complementary type thin film insulating gate field effect transistor (hereafter, cMOSTFT) having an LDD (Lightly Doped Drain) structure having high switching performance and low leak current, and a peripheral drive circuit comprising this cMOSTFT, nMOSTFT, pMOSTFT or a combination thereof, thus realizing a high image quality, high resolution, narrow frame, high efficiency large screen display panel which can be used even with a large glass substrate having a relatively low strain point, permitting high productivity, avoiding costly manufacturing equipment and allowing cost reductions, and further permitting easy adjustment of threshold values, fast operation and larger screen size due to lower resistance.
This invention relates to an electro-optic device and a drive substrate for this electro-optic device comprising a display unit comprising pixel electrodes (e.g., plural pixel electrodes arranged in a matrix, hereafter idem), and a peripheral drive circuit unit surrounding this display unit, these units being disposed on a first substrate (i.e., a drive substrate, hereafter idem), and a predetermined optical material such as a liquid crystal disposed between this first substrate and a second substrate (i.e., a facing substrate, hereafter idem), wherein
a gate comprising a gate electrode and a gate insulation film is formed on one of the surfaces of the first substrate,
a layer of a material with good lattice compatibility with monocrystalline silicon is formed on this surface of the first substrate,
a layer of monocrystalline silicon is formed on the first substrate comprising this material and the gate,
and a dual gate first thin film transistor (in particular, MOSTFT, hereafter idem) comprising this monocrystalline silicon layer as channel region, source region and drain region, and having a gate in the upper part and lower part of the channel region, forms at least part of the peripheral drive circuit unit.
According to this invention, the thin film transistor may be a field effect transistor (FET) (MOS type or junction type, either being satisfactory) or a bipolar transistor, and this invention may be applied to both types of transistor (hereafter, idem).
This invention also provides a method of manufacturing this electro-optic device and drive substrate, comprising:
a process for forming a gate comprising a gate electrode and a gate insulation film on one of the surfaces of the first substrate,
a process for forming a substance layer having good lattice compatibility with monocrystalline silicon on the surface of the first substrate,
a process for heteroepitaxially growing a monocrystalline silicon layer on the first substrate comprising this substance layer and the gate by catalytic CVD or high density plasma CVD using the substance layer as a seed,
a process for performing a predetermined treatment on this monocrystalline silicon layer to form a channel region, source region and drain region, and
a process for forming a first dual gate TFT comprising the gate in the upper part and lower part respectively of the channel region, this thin film transistor forming at least part of the peripheral drive circuit unit.
According to this invention, the monocrystalline silicon layer is formed by heteroepitaxial growth by catalytic CVD or high-density plasma CVD, using the aforesaid substance layer (e.g., a crystalline sapphire film) having good lattice compatibility with monocrystalline silicon as a seed, and this epitaxially grown layer is used for a dual gate MOSTFT of a peripheral drive circuit of the drive substrate such as an active matrix substrate, or a dual gate MOSTFT of a peripheral drive circuit of an electro-optic device such as an LCD comprising a display unit and peripheral drive circuit in a one-piece construction.
(A) By forming the substance layer having good lattice compatibility with monocrystalline silicon (e.g., a crystalline sapphire film), and performing heteroepitaxial growth using this substance layer as a seed, a monocrystalline silicon layer having a high electron mobility of 540 cm2/vxc2x7sec is obtained. An electro-optic device such as a thin film semiconductor used for a display with a built-in high-performance driver can therefore be manufactured.
(B) In particular, this monocrystalline silicon layer demonstrates a high electron and hole mobility characteristic of monocrystalline silicon substrates in comparison to the amorphous silicon layer and polycrystalline silicon layer of the related art.
This monocrystalline silicon dual gate MOSTFT makes it possible to manufacture a display unit comprising a nMOSTFT, pMOSTFT or cMOSTFT having a high switching performance (and preferably, an LDD (Lightly Doped Drain) structure with mitigated electric field intensity to permit low leak current), and a peripheral drive circuit comprising this cMOSTFT, nMOSTFT or pMOSTFT or a combination thereof having high drive performance, in a one-piece construction.
This permits high image quality, high precision, narrow frame, high efficiency, and a large screen display panel. In particular, although it is difficult with polycrystalline silicon to form a pMOSTFT having high hole mobility as a TFT for an LCD, the monocrystalline silicon layer of this invention shows amply high mobility even for holes. Consequently, it is possible to manufacture a peripheral drive circuit driven independently by electrons and holes or by a combination of both, and to realize a panel comprising this drive circuit and a display unit TFT having a nMOS, pMOS or cMOS LDD structure in a one-piece construction.
(C) Further, a dual gate MOSTFT is used for the peripheral drive circuit, so a cMOS, nMOS or pMOSTFT having 1.5-2 times higher drive performance than a single gate type TFT can be formed, and a high drive performance TFT with better functionality is obtained. This is particularly suitable where a high drive performance TFT is required for part of a peripheral drive circuit. For example, this invention is useful not only in that one of a pair of peripheral vertical drive circuits can be omitted, but also because it can be applied to electro-optic devices other than LCD such as organic EL or FED. Further, the dual gate structure can easily be modified to a top gate or bottom gate structure depending on the choice of upper and lower gates. Another advantage is that, even if the upper or lower gate no longer functions, the other gate can be used.
(D) Moreover, as the monocrystalline silicon layer is formed by a low temperature growth technique wherein the aforesaid substance layer is used as a seed for heteroepitaxial growth, and catalytic CVD (chemical vapor deposition using a catalyst, substrate temperature 200-800xc2x0 C. and particularly 300-400xc2x0 C.) is performed on this substance layer, the monocrystalline silicon layer can be formed uniformly at low temperature on the substrate. Hence, it is possible to use a substrate which is easily procured, economical and has good physical properties such as a glass substrate which has a relatively low strain point or a heat resistant organic substrate, and larger substrates may also be employed.
(E) Annealing for long periods at intermediate temperatures (approximately 600xc2x0 C. for 10 hours or more) as in the case of solid phase growth, or excimer laser annealing, are Ad unnecessary. Therefore, productivity is high, costly manufacturing equipment is not required, and cost reduction can be achieved.
(F) In this heteroepitaxial growth, a monocrystalline silicon layer having a wide range of P type or N type electrical conduction and high mobility can easily be obtained by adjusting the crystallinity of the substance layer such as the crystalline sapphire film, the gas composition ratio during catalytic CVD, the substrate heating temperature and the cooling rate. Hence, adjustment of Vth (a threshold value) is easy, and fast operation is possible due to lowering of resistance.
(G) If a suitable amount of a Group III or Group V impurity element (boron, phosphorus, antimony, arsenic, bismuth or aluminum) is doped from a doping gas during film-forming of the monocrystalline silicon by catalytic CVD or a similar process, the type and/or concentration of the impurity in the monocrystalline silicon layer formed by heteroepitaxial growth, i.e., the electrical conductance (P type/N type) and/or carrier concentration, may be controlled as desired.
(H) The aforesaid substance layer such as the crystalline sapphire film forms various atomic diffusion barriers, so diffusion of impurities from the glass substrate can be suppressed.